Inkjet printing is a well-known technique that involves printing without the printing device contacting the printing substrate (non-impact printing). In the present invention we refer to “Drop On Demand” technology where ink drops are created only when required either thermally (thermal technology) or mechanically (piezo or valve technology) and jetted on the substrate forming small dots; the substrate can be very different, ranging from paper to plasticized substrates such as labels or vinyl or polyolefin based or coated substrates. During the printing step, the media can be heated to temperatures of up to 80° C.
Because of this peculiar printing technique, inkjet inks have technical requirements quite different from the traditional inks; more particularly, inkjet inks require very accurate control of viscosity and surface tension and sometimes of electrical conductance.
Electrical conductance is a fundamental parameter especially in Continuous Inkjet technology where drops are generated continuously and then switched to print using electrostatic devices: unlike in prior art, in the present invention electrical conductance has been considered an important parameter also in “Drop On Demand” technology.
Solvent and solvent-less ink formulations are known for ink jet printing. As is well known, in a solvent-based formulation the solvent does not react to provide a final polymer, but evaporates without entering the final dry ink formulation; contrary to this, in radiation curable ink formulations the solvent is replaced by oligomers that are cured to provide a polymer that is part of the dry ink. An example of curable, solvent-less ink formulation is disclosed in WO 03/027162, where the UV curable composition comprises a curable dispersant.
The invention formulations are solvent-based, i.e. they are substantially free from radiation curable oligomers that act as a solvent medium and the filming of the ink is obtained by evaporation of an organic solvent. The invention formulation are especially designed for inkjet printing onto a wide variety of non-porous substrates such as plastic substrates (especially vinyl substrates), but are also suitable for printing on metal, glass, ceramic, rubber and others. These ink formulations can, however, also be used onto porous media like paper and wood. Media to be printed can be either uncoated or coated with special coatings and primers
Within known solvent-based inks there several problems deriving from the choice of the solvent system to be used. While water is a good solvent as far as it regards environment and hazard problems, it also is a source of drawbacks deriving from the use of water as a major solvent when paper is used as a substrate, namely paper cockle and curl, and color bleeding.
EP 0509688 discloses a water based ink formulation comprising a maximum amount of 20% of an organic solvent having low vapour pressure, 0.1 to 10 wt % of a water-soluble dye and a vehicle, the balance being water. Colour bleeding is said to be avoided by using self aggregating components such as surfactants, that can generate micellae; the application is silent about paper cockle and curl problems, that in fact were not solved.
A further problem is that water based inks have very poor performances on non-porous substrates. Water-based inks have poor adhesion on non-porous media because water has no solvency power for non-porous materials such as PVC and other plastic materials, while organic solvents have it: this means that organic solvent-based inks offer a better adhesion on these substrates than the water-based ones.
This problem became a major problem with the increase of articles to be printed on non-porous, plasticized or plastic/vinyl coated articles such as posters for outdoor advertising.
It was proposed to use organic solvents and co-solvents (diluents and thinners) to replace water, in part or completely, in the ink formulation.
WO 98/13430 discloses ink jet ink compositions comprising a binder resin that is suitable for use with ethanol or acetone, that are the preferred solvents. The disclosed formulations are all containing about 80-85 wt % of ethanol (flash point 13° C.) and/or acetone (flash point −20° C.) and are therefore highly flammable formulations. Thus, this solution proved to be not satisfactory because the amount of organic solvents required resulted in ink formulations that are classified as hazardous, often flammable and sometimes even toxic. Such ink formulations are therefore subject to the strict rules for handling said materials.
It was also proposed to use “oils” (e.g. long chain glycols, ethoxylated glycols, ethoxylated oils, ethoxylated fats, ethoxylated fatty acids, ethoxylated alcohols and hydrocarbons) to completely or partially replace water in the ink formulation; however, this solution proved to be of reduced practicity: even if these oil-based or oil-containing inks are usually non-hazardous and non-flammable and reliable in printing process (good head/nozzle stability, reliable running, minimal printer maintenance), they have insufficient versatility. In fact they are only intended for use on partially or totally absorbent substrates, while their use is not recommended on non-porous or glossy media.
U.S. Pat. No. 5,407,474 discloses an ink formulation for non porous substrates; the formulation is comprising a flux and is designed for use on glass and ceramic only and the ink has to be subjected to firing to become permanent. Thus, the ink formulation of this document has the drawback that it cannot be used on other substrates than glass and ceramic (PVC cannot be fired in an oven) and that the presence of a flux is detrimental to the ink viscosity. The solvent is selected from water, alcohol, methylethylketone and their mixtures and therefore this document relates to an ink having the same problems that are discussed above.